Method and apparatus for filtering interference and nonlinear distortions

ABSTRACT

Interference and/or nonlinear distortions are reduced in a signal communicated from a transmitter to a receiver. To reduce interference, the transmitter is momentarily disrupted, during which time the receiver analyzes interference on the communication path to determine the frequency of at least one peak thereof. Information is communicated from the receiver to the transmitter identifying the frequencies of the interference peaks. Based on this information, the transmitter pre-distorts the signal to accentuate the signal magnitude at the identified frequencies. The pre-distorted signal is then communicated to the receiver, where it is filtered to attenuate the signal magnitude at the identified frequencies. An adaptive scheme periodically determines the interference peak frequencies. For nonlinear distortion cancellation (e.g., CSO/CTB), there is no need to disrupt the transmitter since the frequencies of the distortion are already known. Pre-distortion at the transmitter and filtering at the receiver are performed at the known, fixed frequencies.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to electronic communicationssystems, and more particularly to a method and apparatus for filteringinterference and nonlinear distortions in a signal communicated from atransmitter to a receiver via a communication path.

[0002] The invention is particularly suited for use in connection with atelevision distribution system, such as a hybrid fiber/coax (HFC)network, in which a subscriber terminal such as a set-top box or cablemodem receives television and/or data signals from the distributionsystem “headend” via a “downstream” communication path and sendsinformation back to the headend on an “upstream” return path. In such anenvironment, the interference filtered by the present invention is oftenreferred to as “ingress” or “ingress noise” and the nonlineardistortions of concern comprise composite second order (CSO) andcomposite triple beat (CTB) distortions.

[0003] Hybrid fiber/coax networks, which are based on branch and treearchitecture, provide a cost-effective means for delivering downstreambroadband services such as analog/digital video and high-speed data. Inaddition, they provide subscribers with high-speed data upstreamtransmission, for example, in the 5-42 MHz portion of the RF spectrum.Cumulative ingress noise is the main impairment in the return-pathportion of HFC networks. The types of ingress noise, which appear on thereturn-path, can be classified as follows:

[0004] A. Narrowband short-wave signals, originating from radio stationsand other sources, coupled to the return-path cable plant at thesubscriber location or in the distribution plant.

[0005] B. Common mode distortion originating from non-linearities in thecable plant.

[0006] C. Location specific interference generated by an electricaldevice at the subscriber location. See, e.g., C. A. Eldering, N.Himayat, and F. M. Gardner, “CATV Return Path Characterization forReliable Communications”, IEEE Communications 8, 62-68 (1995). Theamount of cumulative ingress noise in the return-path network isessentially the limiting factor in determining the maximum number ofsimultaneous users and the maximum data transmission rate that can beachieved.

[0007] Video signals sent to set-top boxes are also often subject to“burst/impulse noise” originating from peak Composite Second Order (CSO)and/or Composite Triple Beat (CTB) distortions. These distortions aregenerally present at known frequencies, which depend upon the televisionfrequency plan used for the analog video signals. In cable televisionsystems, such frequency plans include the integrally-related-carrier(IRC) plan and the harmonically-related-carrier (HRC) plan. CSO and CTBdistortions can lead to video blocking and visually degraded areas in atelevision picture. One method for addressing the CSO/CTB distortionproblem in multi-channel AM-VSB (amplitude modulated vestigialsideband)/QAM (quadrature amplitude modulation) video transmissionsystems and the like is disclosed in copending, commonly assigned U.S.patant application No. 09/170,852 filed Oct. 13, 1998 and entitled“Method and System for Enhancing Digital Video Transmission To A Set-TopBox.” In the system disclosed in that patent application, theperformance of hybrid analog and digital video transmission systems isimproved by determining the relative magnitude and frequency locationsof nonlinear distortions, identifying the analog channel frequency plan,and then selecting a digital channel map based thereon.

[0008] It would be advantageous to have a robust and cost-effectivemethod and apparatus for filtering interference (such as ingress noiseor other interference types) and nonlinear distortions in a signalcommunicated from a transmitter to a receiver via a communication path,such as a return path signal from a set-top box or the like. It would befurther advantageous to provide such a method and apparatus that operateadaptively, so that interference (e.g., ingress) is efficiently filteredeven when the frequency of the interference peaks changes over time.Such a method and apparatus should enable a plurality of interferencepeaks to be filtered, and should automatically adapt to changingconditions in the interference.

[0009] It would be still further advantageous to provide a method andapparatus for filtering nonlinear distortion in a signal communicatedfrom a transmitter to a receiver via a communication path. Such a methodand apparatus would be particularly useful in the downstream channel ofan HFC cable television distribution system, where either an IRC or HRCfrequency plan is used.

[0010] The present invention provides methods and apparatus enjoying theaforementioned and other advantages.

SUMMARY OF THE INVENTION

[0011] In accordance with the present invention, a method and apparatusare provided for filtering interference in a signal communicated from atransmitter to a receiver via a communication path. The transmitter ismomentarily disrupted from transmitting over the communication path,e.g., by placing it in an idle state. During the momentary disruption,the receiver analyzes interference on the communication path todetermine the frequency of at least one noise peak of the interference.Information is communicated from the receiver to the transmitteridentifying the frequency of the at least one interference noise peak.Based on this information, the transmitter pre-distorts the signal toaccentuate the signal magnitude at the identified frequency orfrequencies of the interference peak(s). The pre-distorted signal isthen transmitted by the transmitter to the receiver, which filters thepre-distorted signal to attenuate the signal magnitude at the identifiedfrequency or frequencies.

[0012] In an illustrated embodiment, the receiver performs a real orcomplex signal frequency analysis on the interference to determine thefrequency peak(s) thereof. The filtering at the receiver can use, forexample, a transfer function that is the inverse of the transferfunction used to pre-distort the signal at the transmitter. In onepossible implementation, the filtering at the receiver uses theZ-transform transfer function:${H(z)} = \frac{1 + {2{{Re}(\alpha)}z^{- 1}} + z^{- 2}}{1 - {2{{Re}(\alpha)}{R \cdot z^{- 1}}} + {R^{2} \cdot z^{- 2}}}$

[0013] where α=exp(2jπφ), φ is the normalized center frequency of thefilter, and R is a constant. The pre-distortion at the transmitter canimplement the inverse transfer function H(z)⁻.

[0014] A power threshold detection can be used during the analysis toidentify the frequency location(s) of the interference peak(s). Forexample, only peaks exceeding a predefined power threshold level mightbe identified for pre-distortion at the transmitter and subsequentfiltering at the receiver.

[0015] In an adaptive method, the transmitter is periodically disruptedfrom transmitting over the communication path. Interference on thecommunication path is analyzed at the receiver during the periodicdisruptions, and information is communicated from the receiver to thetransmitter identifying changes in the interference peak(s) determinedduring the periodic disruptions. The transmitter then pre-distorts thesignal to accentuate the signal magnitude in accordance with the changesof the interference peaks.

[0016] Also disclosed are a method and apparatus for filtering nonlineardistortion in a signal communicated from a transmitter to a receiver viaa communication path. The signal is pre-distorted at the transmitter toaccentuate the signal magnitude at a fixed frequency where the nonlineardistortion takes place. The pre-distorted signal is transmitted to thereceiver, which provides filtering to attenuate the signal magnitude atsaid fixed frequency.

[0017] If the signal is, for example, an integrally-related carrier(IRC) television channel signal having composite second order (CSO) andcomposite triple beat (CTB) distortions present at different fixedfrequencies, the CSO and CTB distortions are filtered by pre-distortingthe signal at the transmitter to accentuate the signal magnitude at afirst fixed frequency where the CSO distortion resides, and at a secondfixed frequency where the CTB distortion resides. The pre-distortedsignal is then filtered at the receiver at the first and second fixedfrequencies.

[0018] If the signal is, for example, a harmonically related carrier(HRC) television channel signal having composite second order (CSO) andcomposite triple beat (CTB) distortions present at a common fixedfrequency, the CSO and CTB distortions are filtered by pre-distortingthe signal at the transmitter to accentuate the signal magnitude at thecommon fixed frequency. The pre-distorted signal is then filtered at thereceiver at the common fixed frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a block diagram of illustrating a cable televisionheadend or cable modem termination system (CMTS) with a return pathreceiver;

[0020]FIG. 2 is a more detailed block diagram of the return pathreceiver component of FIG. 1;

[0021]FIG. 3 is a detailed block diagram of one embodiment of a burstreceiver that can be used in the receiver of FIG. 2 in accordance withthe invention;

[0022]FIG. 4 is a detailed block diagram of an alternate embodiment of aburst receiver that can be used in the receiver of FIG. 2 in accordancewith the invention;

[0023]FIG. 5 is a block diagram of an example second-order notch filterstructure that can be used in a receiver in accordance with theinvention;

[0024]FIG. 6 is a graph illustrating a simulated baseband 256-QAMspectrum with a narrowband interference peak located at 0.5-MHz off thecenter of the channel with C/(N+I)=0dB;

[0025]FIG. 7 is a graph illustrating a simulated baseband 256-QAMspectrum of a pre-distorted signal to be transmitted in accordance withthe invention;

[0026]FIG. 8 is a graph illustrating the simulated 256-QAM basebandspectrum of the signal of FIG. 7 after the interference has been removedby filtering at the receiver in accordance with the invention;

[0027]FIG. 9 is a graph illustrating the amplitude response of a notchfilter that can be used at the receiver for filtering the pre-distortedsignal;

[0028]FIG. 10 is a graph illustrating the phase response of a notchfilter that can be used at the receiver for filtering the pre-distortedsignal;

[0029]FIG. 11 is an illustration of a simulated quadrant of a 256-QAMI/Q constellation with narrowband interference that has not beenfiltered in accordance with the invention; and

[0030]FIG. 12 is an illustration of a simulated quadrant of a 256-QAMI/Q constellation with narrowband interference that has been filtered inaccordance with the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0031] The present invention provides techniques for filtering bothinterference and nonlinear distortions in a communication system. Forexample, the invention is useful to filter ingress noise which mayinclude, for example, narrowband interference, common mode distortionand location specific interference in an upstream channel (e.g., returnpath) of a cable television system. The invention may also be used tofilter nonlinear distortions such as CSO and CTB distortions, which may,for example, be present in the downstream path of a cable televisionsystem. Additional uses of the invention for other interference anddistortion types will be apparent to those skilled in the art.

[0032]FIG. 1 illustrates, in simplified block diagram form, a cablemodem termination system (CMTS) at a cable television headend. The CMTSis controlled by a computer processor (CPU) 10 which communicates withthe other CMTS components over a bi-directional bus coupled to a mediaaccess controller (MAC) 12. The MAC 12 controls the physical layer ofthe communication signals and coordinates various aspects of the datacarried by the signals such as the time data is sent, the time data isreceived, etc. The MAC 12 also receives signals from a return pathreceiver 20 which, in turn, receives signals from a remote subscriberterminal (e.g., a cable modem or set-top box). The data signals to betransmitted are provided by the MAC to a QAM modulator 14 for modulationin a conventional manner. An upconverter 16 converts the output of theQAM modulator to a suitable radio frequency (RF) for transmission over,e.g., a bi-directional HFC network 22. The CMTS is coupled to the HFCnetwork via a diplexer 18 in a conventional manner.

[0033] The front-end of the return-path receiver 20 converts a burstyanalog signal received from the subscriber terminal to a sampled digitalsignal, which can be fed to an off-line (i.e., non-real time) DigitalSignal Processing (DSP) chip 38 in the receiver, as illustrated in FIG.2. In order to filter interference (e.g., narrowband interference suchas ingress), the interference must first be detected. To do this, aninitialization process is commenced wherein the digital transmitter(e.g., QPSK or QAM) in the cable modem (CM) or at the set-top box isplaced in an idle mode, so that only the cumulative interference will bereceived by the return path receiver 20 at the CMTS. The off-line DSP 38provided within the return path receiver 20 analyzes the received noise.This analysis can be performed using a complex signal frequency analyzerwhich uses, for example, a Discrete Fourier Transform (DFT) algorithm.The spectral power density of the interference is significantly higherthan that of the white Gaussian noise (WGN), and the noise peaks caneasily be identified using a threshold power detector. The thresholdlevel can be set, for example, to be 10-dB higher than the WGN floor toassure that only large interference peaks are being identified. Toachieve, for example, 10-kHz interference peak resolution in a 3.2-MHzdata channel (worst-case), 640 sampling points are required for the DFT.

[0034] Once the frequencies of the interference peaks have beendetermined, information identifying these specific frequencies istransmitted to the subscriber terminal via the downstream QAM modulator14, upconverter 16 and diplexer 18 as shown in FIG. 1. It is noted thatalthough a QAM modulator is shown for purposes of illustration, anysuitable form of digital modulation can be used to transmit theinterference peak identification information to the subscriber terminal.As an alternative, an alternate channel (which can be analog or digital)could be used to pass this information to the subscriber terminal.

[0035] Relevant portions of one possible implementation of the returnpath receiver 20 are illustrated in block diagram form in FIG. 2. It isnoted that the illustrated portions are provided as an example only, andthat other implementations will be apparent to those skilled in the art.In a cable television implementation, the receiver portions illustratedcan be provided in a CMTS or other headend embodiment.

[0036] The HFC network 22 is coupled to a tuner 30 provided in thereceiver. A desired upstream signal received from the HFC network, suchas a data signal, is tuned using tuner 30 and passed to an analog todigital converter (A/D) 32. The digitized signal from A/D 32 is passedto an appropriate receiver, such as burst receiver 36 and a DSP 38. TheDSP performs a real or complex signal frequency analysis of the returnpath signals to determine the frequency of each of the interferencepeaks. The DSP sends information indicative of the frequency of eachpeak (e.g., the filters, coefficient data) to the microprocessor 40 toset up notch filters in the burst receiver 36. This information is alsocommunicated to the subscriber's set-top box or cable modem for use insetting up complementary pre-distortion filters. The purpose of thepre-distortion filters in the subscriber's cable modem or set-top box isto accentuate the transmitted return path (upstream) signal at thefrequencies where the interference is expected to occur at the receiver.Corresponding notch filters at the CMTS will then attenuate the samefrequencies at the return path receiver, thereby effectively filteringout the effect of the interference. The attenuation at the burstreceiver 36 not only filters out the interference; it also returns thesignal level to its proper magnitude at the interference peakfrequencies.

[0037]FIGS. 3 and 4 illustrate further detail for two differentembodiments of a return path receiver using notch filters in accordancewith the present invention. The receivers illustrated in these figurescan be used to provide the functions of return path receiver 20 shown inFIG. 1. In particular, the intermediate frequency (IF) return pathsignal from the tuner 30 of FIG. 2 is input to an A/D converter 32 (FIG.2), which digitizes the signal and passes it to I and Q phase quadraturemixers 52, 54 respectively (FIG. 3 or 4). In the embodiment of FIG. 3,square-root Nyquist filters 56, 58 filter the I and Q signals, and passthem to respective notch filters (NF) combined with ÷4 decimationfilters 60, 62, thereby providing down-sampled I and Q signals that havebeen attenuated back to normal levels at the interference frequencypeaks. In the embodiment of FIG. 4, notch filters 51 are providedimmediately prior to quadrature mixers 52, 54 instead of being combinedwith the decimator filters 61, 63.

[0038] The filtered and down-sampled I and Q signals are passed to afeed forward equalizer (FFE) 64 and decision feedback equalizer (DFE) 66for conventional equalization. A forward error correction (FEC) decoder68 then processes the equalized I and Q signals in a conventionalmanner. Acquisition and tracking loop 70 enables the receiver toproperly acquire and track the received signal, as well known in theart.

[0039] The following difference equation describes a second-orderdigital notch filter design that can be used to implement the invention:

y(n)=b₀ ·x(n)+b ₁ ·x(n−1)+b ₂ ·x(n−2) −a ₁ ·y(n−1)−a ₂ ·y(n−2)

[0040] where:

[0041] x(n) and y(n) are the time-discrete input and output sequences,

b ₂ =b ₀=1, b ₁=2Re (α),

a ₁=−2R ² ·Re(α), a ₂ =R ², and

α=exp(2jπφ)

[0042] φ=the normalized center frequency the notch, and R is a constantrelated to the notch magnitude. Thus, only two coefficients are neededfor this filter.

[0043] The second-order notch filter structure, which is shown in FIG.5, is known as a direct form II structure. The transfer function or theZ-transform of the notch filter 80 is described by the followingequation:${H(z)} = \frac{1 + {2{{Re}(\alpha)}z^{- 1}} + z^{- 2}}{1 - {2{{Re}(\alpha)}{R \cdot z^{- 1}}} + {R^{2} \cdot z^{- 2}}}$

[0044] The pre-distortion filter at the subscriber terminal transmitterhas exactly the inverse transfer function as the second-order notchfilter shown above [H(z)⁻¹]. The pre-distortion filters can, forexample, be inserted after the symbol mapper for a QPSK (quadraturephase shift keyed) or QAM (quadrature amplitude modulation)implementation, and before a programmable transmitter pre-equalizer thatis used to cancel the effects of intersymbol interference (ISI). It isnoted that although a notch filter is described herein for use with thereturn path receiver, other types of filtering may be used instead toprovide the same or similar results.

[0045]FIG. 6 illustrates a simulated baseband 256-QAM spectrum 82 with anarrowband interference peak 83 located 0.5-MHz off the center of thechannel and with C/(N+I)=0dB.

[0046]FIG. 7 illustrates the spectrum 84 of the simulated baseband256-QAM spectrum with the pre-distortion 85 added at the transmitter.

[0047]FIG. 8 shows the simulated baseband 256-QAM spectrum 86 at theupstream receiver after the narrow interference peak 83 andpre-distortion 85 have been removed.

[0048]FIGS. 9 and 10 illustrate the amplitude and phase responses 88, 90respectively, of the second order notch filter of FIG. 5, as a functionof the normalized frequency.

[0049]FIG. 11 shows a simulated single quadrant 92 of the 256-QAMconstellation when the interference peak illustrated in FIG. 6 ispresent. It is clear from this figure that the received signal containsmany errors.

[0050]FIG. 12 shows a simulated single quadrant 94 of the 256-QAMconstellation in the presence of the interference peak, but after theapplication of the interference filtering technique of the presentinvention. The simulation results indicate an error-free reception forthe demodulated 256-QAM signal.

[0051] In order to further refine the present invention, the filteringof interference such as ingress noise can be made adaptive. Inparticular, between upstream burst intervals, (e.g., once every second)the subscriber terminal transmitter may be idled to “quiet” the returnpath. This will enable the CMTS to monitor the ingress peaks on thequieted return path. The off-line DSP chip 38 at the return-pathreceiver in the CMTS can determine if the previously detected ingresspeaks are still present or if there are new ingress peaks. The updatedinformation (parameter α in the transfer function for H(z)) on thefrequency of the current interference peak(s) is then sent to thesubscriber terminal (e.g., set-top or cable modem) transmitter via thedownstream modulator to enable and/or disable appropriate pre-distortionfilters at the return-path transmitter. At the same time, appropriatenotch filters at the return path receiver are enabled and inappropriatenotch filters are disabled.

[0052] A variation of the techniques described above can be applied todownstream signal (e.g., digital video) transmission to subscriberterminals to overcome the effect of nonlinear (CSO/CTB) distortions. Thelocation of CSO and CTB distortions in an HFC network depends on thecable TV frequency plan used for the analog video signals. The two mostwidely used cable TV frequency plans are the integrally-related-carrier(IRC) and harmonically-related-carrier (HRC) plans. In the IRC plan thefirst picture carrier frequency is located at 55.2625-MHz withsuccessive picture carriers located six MHz apart up to 1-GHz. In theHRC frequency plan, the picture carrier frequencies are downshifted1.25-MHz compared with the corresponding picture carriers in IRC plan.The advantage of the HRC plan is that the CSO and CTB distortionproducts fall on the picture carrier, and thus their effect becomesalmost invisible. In the IRC plan, the CSO distortions are located ±1.25-MHz from the corresponding picture carrier frequency, and thus canbecome visible.

[0053] The following Table 1 shows the various options for the IRC andHRC frequency plans: TABLE 1 The location of CSO and CTB distortionsrelative to the QAM channel center frequency. Frequency Plan CSODistortions CTB Distortions IRC Plan 0.5-MHz relative to 1.75-MHzrelative to QAM channel center QAM channel center frequency. frequency.HRC Plan 1.75-MHz relative to 1.75-MHz relative to QAM channel centerQAM channel center frequency. frequency.

[0054] To overcome the impact of CSO/CTB distortions using the IRC planat the QAM receiver, two pre-distortion filters (one for CSO and one forCTB) with H(z)⁻¹ frequency response are enabled in the QAM modulator,which is located at the cable headend. Two notch filters with H(z)frequency response and with the same coefficients are also enabled inthe QAM receiver in the set-top box. No adaptation is required heresince the frequencies of the nonlinear distortions are always known inhybrid analog/digital HFC networks. For the HRC plan, only onepre-distortion filter and one corresponding notch filter are required,since CSO and CTB occur at the same frequency.

[0055] It should now be appreciated that the present invention providesmethods and apparatus for filtering interference and nonlineardistortions in communications systems. Although the invention has beendescribed in connection with cable television systems, wherein thedownstream path may suffer from nonlinear distortions (CSO/CTB) and theupstream path may suffer from ingress noise, the invention is notlimited to use in such systems or to these types of interference anddistortion. The novel techniques for interference detection andreduction and for filtering nonlinear distortion are applicable to anycommunication system where such interference resides at detectable oralready known frequencies. By pre-distorting the transmitted signal andfiltering the pre-distorted signal at the receiver, effective reductionor elimination of interference and nonlinear distortion is obtained.Accordingly, various adaptations and modifications may be made to theinvention without departing from the scope thereof as set forth in theclaims.

What is claimed is:
 1. A method for filtering a signal communicated froma transmitter to a receiver via a communication path to reduceinterference, comprising the steps of: momentarily disrupting saidtransmitter from transmitting over said communication path; analyzinginterference on said communication path at the receiver during themomentary disruption to determine the frequency of at least oneinterference peak; communicating information from the receiver to thetransmitter identifying the frequency of the at least one interferencepeak; pre-distorting said signal at the transmitter to accentuate thesignal magnitude at the identified frequency; communicating thepre-distorted signal to said receiver; and filtering the pre-distortedsignal at said receiver to attenuate the signal magnitude at theidentified frequency.
 2. A method in accordance with claim 1 whereinsaid analyzing step performs a real or complex signal frequency analysison the interference to determine the frequency peak(s) thereof.
 3. Amethod in accordance with claim 1 wherein said filtering at the receiveruses a transfer function that is the inverse of the transfer functionused to pre-distort the signal at the transmitter.
 4. A method inaccordance with claim 1 wherein said filtering at the receiver uses theZ-transform transfer function:${H(z)} = \frac{1 + {2{{Re}(\alpha)}z^{- 1}} + z^{- 2}}{1 - {2{{Re}(\alpha)}{R \cdot z^{- 1}}} + {R^{2} \cdot z^{- 2}}}$

where α=exp(2jπφ), φ is the normalized center frequency of the filter,and R is a constant.
 5. A method in accordance with claim 4 wherein saidpre-distortion at the transmitter implements the inverse transferfunction H(z)⁻¹.
 6. A method in accordance with claim 1 wherein: saidtransmitter is periodically disrupted from transmitting over saidcommunication path; interference on said communication path is analyzedat the receiver during the periodic disruptions; information iscommunicated from the receiver to the transmitter identifying changes inthe interference peak(s) determined during the periodic disruptions; andthe transmitter pre-distorts the signal to accentuate the signalmagnitude in accordance with the changes of the interference peaks.
 7. Amethod in accordance with claim 1 wherein said analysis step identifiesthe frequency location(s) of the interference peak(s) in accordance witha power threshold level.
 8. Apparatus for filtering interference in asignal communicated from a transmitter to a receiver via a communicationpath, said receiver comprising: a real or complex signal frequencyanalyzer adapted to analyze interference on the communication pathduring momentary disruptions of said signal to determine the frequencyof at least one interference peak, and means for communicatinginformation from the receiver to the transmitter identifying thefrequency of the at least one interference peak; said transmittercomprising: a filter adapted to pre-distort said signal at thetransmitter to accentuate the signal magnitude at the identifiedfrequency; and said receiver further comprising: a filter adapted toattenuate the signal magnitude of the pre-distorted signal at theidentified frequency.
 9. Apparatus in accordance with claim 8 wherein:said communication path comprises a cable television system return pathcoupling a subscriber location to a cable television system headend,said transmitter is provided at the subscriber location; said receiveris provided at the cable television system headend; and saidinterference comprises ingress noise.
 10. Apparatus in accordance withclaim 8 wherein: said transmitter communicates the pre-distorted signalto the receiver using digital modulation.
 11. Apparatus in accordancewith claim 8 wherein the filter at said receiver comprises a notchfilter.
 12. Apparatus in accordance with claim 11 wherein said notchfilter has a Z-transform transfer function described by:${H(z)} = \frac{1 + {2{{Re}(\alpha)}z^{- 1}} + z^{- 2}}{1 - {2{{Re}(\alpha)}{R \cdot z^{- 1}}} + {R^{2} \cdot z^{- 2}}}$

where α=exp(2jπφ), φ is the normalized center frequency of the filter,and R is a constant.
 13. Apparatus in accordance with claim 12 whereinthe pre-distortion filter at the transmitter implements the inversetransfer function H(z)⁻¹.
 14. Apparatus in accordance with claim 13wherein: said complex signal frequency analyzer periodically analyzesinterference on the communication path during momentary disruptions ofsaid signal to determine changes in the interference peak(s) over time,and said notch and pre-distortion filters are programmable to providesaid signal attenuation and accentuation, respectively, at theinterference peak(s) as the frequency of the peak(s) changes over time.15. Apparatus in accordance with claim 8 wherein said complex signalfrequency analyzer includes a threshold power level detector for use indetermining the frequency of said at least one interference peak.
 16. Amethod for filtering nonlinear distortion in a signal communicated froma transmitter to a receiver via a communication path, comprising thesteps of: pre-distorting said signal at the transmitter to accentuatethe signal magnitude at a fixed frequency where said nonlineardistortion resides; communicating the pre-distorted signal to saidreceiver; and filtering the pre-distorted signal at said receiver toattenuate the signal magnitude at said fixed frequency.
 17. A method inaccordance with claim 16 wherein: said signal is an integrally relatedcarrier (IRC) television channel signal having composite second order(CSO) and composite triple beat (CTB) distortions present at differentfixed frequencies; and said CSO and CTB distortions are reduced bypre-distorting said signal at the transmitter to accentuate the signalmagnitude at a first fixed frequency where said CSO distortion residesand a second fixed frequency where said CTB distortion resides, andfiltering said signal at the receiver at said first and second fixedfrequencies.
 18. A method in accordance with claim 16 wherein: saidsignal is a harmonically related carrier (HRC) television channel signalhaving composite second order (CSO) and composite triple beat (CTB)distortions present at a common fixed frequency; and said CSO and CTBdistortions are reduced by pre-distorting said signal at the transmitterto accentuate the signal magnitude at said common fixed frequency andfiltering said signal at the receiver at said common fixed frequency.19. Apparatus for filtering nonlinear distortion in a signalcommunicated from a transmitter to a receiver via a communication path,comprising: a first filter at the transmitter to provide a pre-distortedsignal having an accentuated magnitude at a fixed frequency where saidnonlinear distortion resides; and a second filter at the receiveradapted to filter the pre-distorted signal to attenuate the signalmagnitude at said fixed frequency.
 20. Apparatus in accordance withclaim 19 wherein said second filter comprises a notch filter having aZ-transform transfer function described by:${H(z)} = \frac{1 + {2{{Re}(\alpha)}z^{- 1}} + z^{- 2}}{1 - {2{{Re}(\alpha)}{R \cdot z^{- 1}}} + {R^{2} \cdot z^{- 2}}}$

where α=exp(2jπφ) φ is the normalized center frequency of the filter,and R is a constant; and said first filter implements the inversetransfer function H(z)⁻¹.